NEW MATERIAL:A compound of formula I (R1 is acyl; R2 is H, lower alkyl) and its salt.

EXAMPLE: 2(S)-Hydroxy-3(S)-( 4-formyloxyphenyl )-3-( 2-nitrophenylthio ) propionic acid.

USE: A synthetic intermediate of diltiazem hydrochloride, which is a calcium antagonist.

PREPARATION: The addition of hydrochloric acid to a compound of formula IIis effected in a polar solvent such as acetonitrile, preferably in the presence of a weak-acid catalyst to furnish a compound of formula III as well as a small amount of a compound of formula III'. Then, the compound of formula III solely, or a mixture thereof with the compound of formula III' is dried under reduced pressure, dissolved in an organic solvent such as acetonitrile and allowed to react with a compound of formula IV in the presence of a base such as pyridine to give the compound of formula I .